Recently, demand for photographic performance of a silver halide photographic light sensitive material (hereinafter, referred to as photographic material) become severe. In particular, demands for not only basic performance such as high sensitivity, low fog and superior graininess but also other performance such as pressure resistance and storage stability become stronger than those in the past.
In general, silver halide photographic light sensitive materials are subject to a variety of pressure. A negative film, for example, is subject to pressure when being cut or perforated in the manufacturing process thereof, or bent or abraded when being transported in the camera. As well known in the art, when a variety of pressure are applied to the silver halide photographic material, changes in photographic performance are produced, and a technique for enhancing resistance to these pressure has been desired. Popularization of a compact camera and a film-built-in camera leads to daily photographing and spread of its use, so that silver halide photographic materials are held under various environments and used under various conditions. As a result, their storage stability become one of important performance items.
A dominant factor of basic photographic performance of the silver halide photographic light sensitive material concerns silver halide grains, and development of silver halide grains directed to enhancement of sensitivity and image quality has energetically been engaged so far. Generally speaking, it is effective for enhancement of image quality to make smaller the size of silver halide grains, leading to an increase of the number of grains per unit area and the number of color-developed points (i.e., number of image elements).
Making the grain size smaller, however, results in lowering of sensitivity so that it is limitative for satisfying both high sensitivity and high image quality. To achieve further higher sensitivity and higher image quality, there have been studied techniques of enhancing a ratio of sensitivity to size of the silver halide grain and as one of them, a technique of employing tabular silver halide grains are described in JP-A 58-111935 (herein, the expression, "JP-A" is referred to as unexamined and published Japanese Patent Application), 58-111936, 58-111937, 58-13927 and 59-99433.
As compared to regular crystal silver halide grains such as hexahedral grains, octahedral grains, or dodecahedral grains, the tabular silver halide grains each have larger surface area per grain so that, in the case of the same volume, the tabular grains can cause a larger amount of a spectral sensitizing dye to be adsorbed to the grain surface, advantageously leading to further sensitization. There are also disclosed a technique of providing a site with a high iodide content inside the tabular silver halide grain, as described in JP-A 63-92942 and a technique of employing hexagonal tabular silver halide grains, as described in 63-151618, each showing effects in sensitivity and graininess.
JP-A 63-106746 discloses the use of tabular silver halide grains having substantially a layered structure parallel to two major faces which are opposite with each other, and JP-A 1-279237 also discloses the use tabular silver halide grains having a layer structure divided by plane substantially parallel to two opposite major faces, in which the outermost layer thereof has a higher average iodide content by 1 mol % or more than an average overall iodide content of the grains. In addition, JP-A 1-183644 discloses a technique of using tabular silver halide grains in which iodide distribution in the iodide containing silver halide phase is completely uniform.
There are some reports concerning a technique in view of parallel twin planes of the tabular silver halide grains (hereinafter, sometimes referred to as tabular grains). For example, JP-A 63-163451 discloses a technique of using tabular grains having 5 or more of a ratio (b/a) of grain thickness (b) to a longest spacing between two or more parallel twin planes (a). JP-A 1-201649 discloses a technique of limiting the number of dislocation lines present in tabular silver halide grains, describing its effect on sensitivity, graininess and sharpness.
WO No.91/18320 (herein, the term, "WO" means published International Patent Application) discloses a technique of using tabular silver halide grains having a spacing between at least two twin planes of less than 0.012 mm, and JP-A 3-353043 discloses a technique of using core/shell type tabular silver halide grains having an average longest twin plane spacing of 10 to 100 .ANG., each disclosure describing improvements in sensitivity and graininess, or sharpness, pressure characteristics and graininess, respectively.
A technique which is regarded, in the art, as one of the most basic and important techniques in the process of studying of silver halide emulsions for the purpose of enhancing sensitivity and image quality of a silver halide photographic light sensitive material is one of making silver halide emulsion grains monodisperse. Since an optimal condition for chemical sensitization is different between large-sized grains and those with small-sized ones, it is hard to subject a silver halide emulsion which is comprised of both grains, i.e., polydispersed (broad in grain size distribution), to optimal chemical sensitization, often resulting in an increase of fog and insufficient chemical sensitization. In the case of a monodispersed silver halide emulsion, on the other hand, it is easy to subject the emulsion to optimal chemical sensitization, enabling to prepare the silver halide emulsion with high sensitivity and low fog. Furthermore, it is possible to expect a characteristic curve with a high contrast (high gamma).
With regard to a technique of making tabular silver halide grains monodisperse, JP-A disclosed a technique of improving sensitivity and graininess with monodisperse tabular silver halide grains with two parallel twin planes. JP-A 5-173268 and 6-202258 disclose preparation of tabular silver halide grains with narrow grain size distribution. In these techniques of making the tabular grains monodisperse, the monodisperse tabular grains are referred to as those with a narrow distribution with respect to the grain projected area. Further, JP-A 6-258744 discloses improvements in sensitivity, contrast, pressure resistance and latent image stability by use of monodisperse tabular silver halide grains internally having region different in halide composition. Herein, the expression, "monodisperse" means narrow distribution with respect to the volume of the grains. Thus, these conventional techniques concerning monodisperse tabular silver halide grains are to note the projected are diameter and the variation coefficient of grain volume alone, and are not a technique with intent to control a variation coefficient of grain thickness.
With regard to a technique thickness of the tabular silver halide grains, there have been known techniques described in JP-A 6-43605, 6-43606 and 7-191425. More concretely, a technique disclosed in JP-A 6-43605 or 6-43606 is to note an average value of thickness of the tabular silver halide grains and a technique disclosed in JP-A 7-191425 concerns limitation with respect to a ratio of a variation coefficient of grain thickness to a variation coefficient of twin plane spacing.
With respect to making narrow thickness distribution of the tabular grains, the above JP-A 6-43605, 6-43606 and 7-191425 suggest its usefulness in photographic performance and emulsion preparation, but teach no technique for embodiment thereof.
JP-A 173272 discloses a silver halide emulsion comprised of hexagonal tabular silver halide grains having even-numbered twin planes parallel to the major face and a maximum adjacent edge ratio of 2.0 to 1.0, a variation coefficient of grain size being in a range of 21 to 29% and that of grain thickness, 20% or less. In Examples of the disclosure is cited, as a comparative example, a silver halide emulsion containing tabular grains with a variation coefficient of diameter of 20% or less and a variation coefficient of grain thickness of 20% or less. However, these variation coefficients of the emulsion are values measured with respect to hexagonal tabular silver halide grains having a maximum adjacent edge ratio of 2.0 to 1.0. It was proved through the inventor's following this example that hexagonal tabular silver halide grains having major faces with a maximum adjacent edge ratio of 2.0 to 1.0 accounted for about 90% or less of the grain projected area, and further thereto, small grains which appeared to be regular crystals and coarse grains having a plurality of non-parallel twin planes are also present in the emulsion. As a result of measurements of grain diameter and thickness with respect to any grains contained in the emulsion, it was proved that variation coefficients thereof both exceeded 20%.
As a method for enhancing sensitivity of a silver halide emulsion, U.S. Pat. No. 4,956,269 discloses a technique of introducing dislocation lines into tabular silver halide grains. As is generally known, application of pressure to silver halide grains results in fog or desensitization. In particular, silver halide grains into which dislocation lines are introduced have such a problem that, when subjected to pressure, marked desensitization occurs. JP-A discloses a silver halide emulsion, in which at least 50% by number of total tabular grains is accounted for tabular grains having an aspect ratio of 8 or more a ratio (b/a) of grain thickness (b) to a longest spacing between two or more, parallel twin planes (a) of 5 or more, and at least 50% by number of total tabular grains is accounted for by grains having dislocation lines of 10 or more. The disclosure further describes a preferred embodiment in which a variation coefficient of grain thickness is 30% or less and a variation coefficient of projected area is 20% or less.
As a result of the study by the inventor, however, it was shown that the emulsion obtained according to the above disclosure contained, besides the tabular grains, another type of silver halide grains, such as regular crystal grains and non-parallel tabular grains. It was further shown that a variation coefficient of grain size of the obtained emulsion was more than 20%. Thus, the emulsions obtained according to the above disclosure were distinct from emulsions according to the present invention, as described below.
However, a silver halide emulsion having such a feature is not concretely described in the disclosure, and marked pressure desensitization due to introduced dislocation lines has not been improved through the technique taught by the disclosure. JP-A 3-189642 discloses a silver halide emulsion containing tabular silver halide grains having an aspect ratio of 2 or more and dislocation lines of 10 or more in the fringe portion of the grain, the tabular silver halide grains being monodisperse with respect to size distribution. The disclosure, however, is silent with respect to grain thickness distribution.
It is, for example, effective in decreasing the variation coefficient of grain thickness to retard the grain growth in the direction of the grain thickness, through the course of nucleation and growth. More concretely, there are a method in which grain growth in the direct parallel to the major face is accelerated by causing the grain to form at a low pBr, resulting in retardation of the growth in the direction of grain thickness; and a method in which the grain growth in the direction of the grain thickness is retarded by restraining super-saturation during the course of the grain growth. In these methods, however, it was proved that an aspect ratio of the resulting tabular grains increased, resulting in marked increase of the variation coefficient of grain size.
Thus, there has not been obtained a silver halide emulsion containing tabular silver halide grains relating to the present invention, the tabular grains having dislocation lines and variation coefficients of grain size and grain thickness both being smaller. In addition thereto, it has not been known that pressure desensitization of the silver halide grains having dislocation lines can be improved by use of the silver halide emulsion.